The walls of the Innovation Room at the county complex Sept. 24 were covered with lengthy timelines showing decades of efforts by towns from Sandwich to Brewster to grapple with water quality issues. Even so, the work ahead of all Cape towns seems to stretch to the vanishing point.

Nevertheless, the Cape Cod Bay Watershed Working Group convened this week to do its part in advancing an updated Cape Cod 208 Water Quality Plan. A similar group drawn from Barnstable, Mashpee and Sandwich met Sept. 26 at the Centerville-Osterville-Marstons Mills Fire Station to address the Centerville River, Rushy Marsh and Three Bays areas.

Scot Horsley, president of Horsley Witten Group and area manager for both working groups, told the Cape Cod Bay group that they had much to build on. He displayed a 1929 sewering plan for Hyannis and some areas of Craigville.

“We’re still working on these,” he said. “They really weren’t too far off in 1929.”

Groundwater protection work done on Cape Cod has won national and international renown, according to Horsley.

As part of the 208 plan update, 11 working groups Capewide will review baseline conditions in their watersheds, the purpose of this week’s sessions. In October, members will look at technologies that are available and that work on the Cape. In December, they’ll meet again to develop a variety of responses to the need to better control nitrogen in coastal waters, phosphorous in lakes and ponds and perhaps even contaminants of emerging concern.

Barnstable’s representatives on the Cape Cod Bay group included Town Councilor Ann Canedy, DPW Director Dan Santos and West Barnstable Water Commissioner Bill McMahon.

Horsley admitted that the northside planners were working in something of a vacuum as the Massachusetts Estuaries Project draft showing the amount of nitrogen reduction required on the northside won’t be released until November at the earliest.

“This is good science,” Horsley said, “good as anything out there. People have made as significant decisions based on much less data. We have at least three years of water quality data.”

Efforts to date have focused on reducing “controllable” nitrogen levels, including that from stormwater runoff, fertilizer and septic systems. Some members of the group wanted data on all nitrogen deposits, including that from acid rains tainted by pollutants from Midwest industries.

“When a 40-foot whale decides to pull the chain in his toilet, what kind of nitrogen load does that cost?” asked Peter McDowell, a Dennis water commissioner.

Horsley said the MEP data would cover all sources.

Cape Cod Bay’s 9-foot tidal range provides an enormous flushing advantage that southside waters can’t match, and large areas of salt marsh have a role to play in assimilating nitrogen. A new, off-Cape study of how much nitrogen can be absorbed by a marsh before it begins to break down may indicate a limit lower than hoped, however.